How to pack fortran interface as a standalone module - fortran

Question

I want to package the fortran interface as a standalone library, something similar head file in c/c++. Here is my code. The problem is that i want to call the "hello" subroutine to run the "hello" subroutine. can anyone help me out?
I want to put Interface and set_callback,invoke_callback as a standalone module file.
MODULE DEMO
INTERFACE
SUBROUTINE callback_prototype(callbackID)
CHARACTER(*) :: callbackID
END SUBROUTINE callback_prototype
END INTERFACE
PROCEDURE( callback_prototype ), POINTER :: f_ptr => NULL()
CONTAINS
SUBROUTINE set_callback(func)
IMPLICIT NONE
EXTERNAL :: func
f_ptr => func
call HELLO
END SUBROUTINE
SUBROUTINE invoke_callback(callbackID)
CHARACTER(*) :: callbackID
if (associated(f_ptr)) call f_ptr(callbackID)
END SUBROUTINE
SUBROUTINE HELLO
IMPLICIT NONE
Dosomething
END SUBROUTINE
END MODULE

Answer 1

This would be easily possible with submodules, unfortunately they are not widely available. Thus, I guess the current solution would be to use "hello" as an external function with an implicit interface and putting it into a different file.
Update: Some example based on your code.
subroutine HELLO
implicit none
write(*,*) 'HELLO'
end subroutine HELLO
module demo
implicit none
interface
subroutine callback_prototype(callbackID)
character(*) :: callbackID
end subroutine callback_prototype
end interface
procedure(callback_prototype), pointer :: f_ptr => NULL()
contains
subroutine set_callback(func)
procedure(callback_prototype) :: func
external :: HELLO
f_ptr => func
call HELLO()
end subroutine set_callback
subroutine invoke_callback(callbackID)
character(*) :: callbackID
if (associated(f_ptr)) call f_ptr(callbackID)
end subroutine invoke_callback
end module demo
program test_demo
use demo
implicit none
write(*,*) 'small program'
end program test_demo
Putting this into a file test.f90, and running "gfortran test.f90" produces an executable. Of course, if you want to have that stuff separated you would rather put each part in separate files. You then have to ensure, that the object file containing "hello" is included during link time. And as pointed out by Vladimir_F, you can use an interface declaration to equip the routine with an explicit interface, where you are calling it.
2nd update:
If you put these into three different files
hello.f90
demo.f90
test.f90
You should be able to compile an executable out of them like this:
gfortran -c demo.f90 hello.f90 test.f90
gfortran *.o
The first line creates the object files, while the second links them together. Note, that order is important, the module information for demo.f90 needs to be available before compiling test.f90.